Optimum utilization of high capacity machine tools such as milling machines requires that the work table be provided with interchangeable work supporting pallets and a pallet changer so that as soon as a machining operation is completed, a finished work piece fixtured on a pallet may be removed from the machine and a new work piece fixtured on a second pallet may be placed in the machine in the shortest possible time.
One of the absolute essentials of a machine tool work table which uses pallets is that the pallets must be firmly and accurately locked to the work table; and in the case of an indexing work table it is necessary that indexing of the table take place without disturbing the orientation of the pallets on the table. Positive indexing is customarily accomplished by the use of an annular toothed coupling which is preferably a Curvic coupling because of the particular tooth conformity of such couplings which causes a table to assume precisely the same angular and lateral location in spite of wear in the coupling teeth. The work table is arcuately indexed relative to the traversing base by elevating it sufficiently to disengage the coupling teeth and rotating it to the desired extent for the next operation.
However, where the indexing table carries a pallet, rather than directly supporting the workpiece, the need for precise positioning of the pallet with reference to the rotary indexing member has commonly resulted in the use of two Curvic couplings or suitable equivalents, with a first such coupling being used to locate the pallet with respect to an indexable mounting plate, and with the second being used to positively fix the position of the mounting plate relative to the traversing table base.
By reason of the foregoing structure, it has previously been necessary to provide two interfaces between the traversing table and the pallet, one such as a Curvic coupling between a pallet and a mounting plate, and another between the mounting plate and the work table base. This requires relatively complex mechanism.
Another problem in the use of Curvic couplings with pallet carrying indexing type work tables is the protection of the exposed, upwardly open coupling teeth from the time a pallet is removed until another pallet is in position and the coupling teeth are meshed.